Add xfail_if_raises context manager to defer xfail condition evaluation

The @pytest.mark.xfail decorator with raises=... evaluates its condition
during test collection, which can trigger external calls (e.g.,
Git().config()) that have side effects. Introduce xfail_if_raises as a
context manager that delays this evaluation until test execution time,
and apply it to test_index_mutation.

Co-authored-by: OpenCode <ai-agent@example.invalid>

Author: elovelan

test/lib/helper.py

@@ -35,8 +35,10 @@
 import time
 import unittest
 import venv
+from typing import Union
 
 import gitdb
+import pytest
 
 from git.util import rmtree, cwd
 
@@ -465,3 +467,27 @@ def _executable(self, basename):
         if osp.isfile(path) or osp.islink(path):
             return path
         raise RuntimeError(f"no regular file or symlink {path!r}")
+
+
+@contextlib.contextmanager
+def xfail_if_raises(
+    condition: bool,
+    *,
+    raises: Union[type[BaseException], tuple[type[BaseException], ...]],
+    reason: str = "",
+    strict: bool = False,
+):
+    """Approximates the behavior of @pytest.mark.xfail(..., raises=...) as a context
+    manager that can be used within a test, such as when the condition is complex or has
+    side effects
+
+    One difference is it will not report XPASS if the test passes, but setting `strict`
+    simulates it by raising an exception"""
+    try:
+        yield
+    except raises:
+        if condition:
+            pytest.xfail(reason)
+        raise
+    if strict and condition:
+        pytest.fail("[XPASS(strict)] " + reason)